1. Field of the Invention
The invention is concerned with preserving the aesthetic appearance of architectural structures, e.g. buildings, freestanding walls, statuary and asphalt roofing shingles. More specifically, the invention concerns the treatment of building materials to prevent or remove disfiguring growths such as dark streaks of algae.
2. Description of the Related Art
Concrete provides durable, inexpensive building facings, roofing tiles and statuary, but can quickly develop dark streaks due to algae growth. Other building materials such as wood, limestone and other stonework, masonry, glass and asphalt roofing shingles likewise can become streaked by algae. The discoloration of asphalt roofing shingles is particularly noticeable when their roofing granules have a light color.
Concrete, stonework and masonry can be coated with silicones or silicates for water repellency, but such treatments do not prevent algae and other growths. Instead, it has been necessary to clean concrete periodically, usually with bleaches or phosphates, to remove algae growth. The cleaning operation is time-consuming and expensive and does nothing to inhibit recurring growth.
Algae growth on the granules of asphalt roofing shingles can be virtually eliminated by incorporating copper into the silicate binder that holds the color to the roofing granules. However, doing so substantially increases the cost of manufacturing the shingles. If copper could be incorporated into concrete, it might likewise inhibit algae growth, but copper would interfere with the setting up of the concrete. Another way to minimize algae growth on asphalt shingles is to minimize its content of limestone filler, but this can be economically undesirable.